Zero error in case of screw gauge

While taking a reading, the thimble is turned until the wire is held firmly between the anvil and the spindle.

The least count of the micrometer screw can be calculated using the formula given below:

Least count 

= 0.01 mm

Types of error in micrometer screw gauge reading

Every micrometer prior to its use should be thoroughly checked for backlash error or zero error.

a.Backlash error: Sometimes due to wear and tear of the screw threads, it is observed that reversing the direction of rotation of the thimble, the tip of the screw does not start moving in the opposite direction immediately, but remains stationary for a part of rotation. This is called back lash error.

b.Zero error: If on bringing the flat end of the screw in contact with the stud, the zero mark of the circular scale coincides with the zero mark on base line of the main scale, the instrument is said to be free from zero error. Otherwise an error is said to be there. This can be both positive and negative zero error.

Calculating micrometer screw gauge reading: 
Total observed reading = main scale reading + (circular scale division coinciding the base line of main scale) x least count

True diameter = observed diameter – zero error

Example, main scale reading = 2mm or 0.2cm

Circular scale reading = 56, so 56 x 0.001 = 0.056cm

So observed reading = 0.2 + 0.056 = 0.256cm